Ink jet printing is a non-impact printing process in which the printer produces droplets of ink in response to digital signals, such as those generated by a computer. The droplets of ink are deposited on a substrate medium such as paper or transparent films. Ink jet printers have found broad commercial acceptance due to their print quality, low cost, relatively quiet operation, and graphics capability. Thermal (bubble jet) and piezoelectric drop-on-demand printers have been especially successfiul in the marketplace and have found broad application as printers for personal computers in the office and the home.
Three major concerns dominate inkjet technology: (1) reliability, (2) drying rate, and (3) print quality. The first two concerns, reliability and drying rate, are strongly influenced by cosolvents which are included in the aqueous ink compositions. Ifthe drying rate is ignored, print quality is also influenced by cosolvents. However, for ink jet inks with relatively fast drying rates, print quality is only slightly influenced by the addition of cosolvents. As the present invention relates to aqueous ink compositions, which have relatively fast drying rates and which contain novel compounds as cosolvents, the advantages of the novel compounds with respect to reliability and drying rate are emphasized. Typically, reliability is evaluated with respect to the following four criteria. The first is robustness with respect to continuous printing conditions such that the ink droplet weight does not change over time and good directionality is maintained. Good directionality means that the angular deviation of an ejected ink droplet from a nozzle is within about .+-.0.5.degree. from the normal to the plane of the nozzle. The second is robustness with respect to intermittent printing conditions such that the nozzles do not clog over the time interval in which printing is discontinued. The third is robustness with respect to long term storage of the ink within the print head such that printing behavior unchanged from the original printing behavior (ink droplet weight and good directionality) can be restored after applying a limited amount of suction to the nozzles. The fourth is chemical and physical stability of the ink towards storage at two temperature extremes and towards cycling between those two temperature extremes for an extended period of time. The drying rate of the ink is an important factor in determining the throughput rate of the printer. In a sheet fed printer, the ink on a printed sheet must be dry before the succeeding sheet contacts it. If the ink is not dry, smearing will occur.
Robustness with respect to continuous printing, robustness with respect to intermittent printing, and robustness with respect to long term storage of the ink within the print head are generally highly correlated. Failure to achieve the necessary robustness is often due to evaporation of solvent(s) causing precipitation and/or crystallization of solids at the air/liquid interfaces, which exist in every nozzle on the print head. For inks which contain dispersed pigments as the colorant, failure to achieve the necessary robustness may also be caused by flocculation of the dispersed pigments due to a change in solvent composition as solvent or cosolvents evaporate. Typical cosolvents which contribute to achieving the necessary robustness with respect to continuous printing, intermittent printing, and long term storage of the ink within the print head are typically classified as humectants. Humectants have high affinities for water and relatively high boiling points. A necessary property, but not a sufficient property, for a humectant cosolvent is a boiling point greater than about 230.degree. C.
Chemical and physical stability of the ink towards storage at two temperature extremes and towards cycling between those two temperature extremes is relatively easy to achieve for inks in which water-soluble dyes are used as the colorant. For inks which contain dispersed pigments, many commonly used cosolvents will cause destabilization of typical pigment dispersions, especially at the high temperature extreme.
Formulating ink compositions which dry sufficiently rapidly on a variety of media is well known in the art. Rapid drying is achieved by having the ink composition penetrate into the interior of the print medium. To facilitate penetration, media penetrating cosolvents, which are known in the art as penetrants, are included in the ink composition. Typical penetrant cosolvents have separate hydrophobic and hydrophilic parts.
The two concerns, reliability and drying rate, are often in conflict. As a result, it is difficult to obtain both reliability and a sufficient drying rate with a single additive. In general, cosolvents which are advantageous with respect to obtaining good reliability are poor penetrants. In contrast, cosolvents which are good penetrants are, in general, not advantageous with respect to obtaining good reliability. Accordingly, a need exists for cosolvents which are good penetrants and which are not deleterious to ink reliability.
In Japan Laid-Open Patent Application 62-84171, an aqueous recording solution, which contains either a cyclic acetal, derived from glycerol and an aldehyde, or a cyclic ketal, derived from glycerol and a ketone, is described. The aldehydes and ketones of that invention are hydrocarbon based and contain only carbon, hydrogen, and oxygen. Enumerated aldehydes include the following hydrocarbon aldehydes: formaldehyde, acetaldehyde, propionaldehyde, butyraldehyde, isobutyraldehyde, benzaldehyde, etc. Enumerated ketones include the following hydrocarbon ketones: acetone, 2-butanone, 3-pentanone, cyclohexanone, 4-methyl-2-pentanone, d-camphor, acetophenone, diacetone alcohol, etc. In comparison to a dye-based ink containing 5% ethylene glycol and 10% glycerol, the example inks of Japan Laid-Open Patent Application 62-84171 containing cyclic acetals or cyclic ketals are faster drying. However, in comparison to current best practice inks which contain known penetrants, such as Butyl Carbitol (R) (diethylene glycol mono-n-butyl ether), both the example inks and that comparison ink would be considered slow drying inks.
In comparison to a dye-based ink containing 15% polyethylene glycol #300, the example inks of Japan Laid-Open Patent Application 62-84171 have better chemical and physical stability towards storage at 60.degree. C. However, in comparison to current best practice inks, both the example inks and that comparison ink would lack the necessary robustness, with respect to continuous printing, intermittent printing, and long term storage of the ink within the print head, needed for actual use in a commercial printer. In summary, in comparison to cosolvents used in current best practice inks, the compounds claimed in Japan Laid-Open Patent Application 62-84171 are neither particularly good humectants nor particularly good penetrants.
In U.S. Pat. No.3,741,986, a process is described for preparing cyclic ketals based upon (1) polyhydric alcohols containing at least three hydroxyl groups and at least three carbon atoms and (2) ketones wherein each alkyl radical is polyhalogenated. The cyclic ketals of that invention have proposed applications as fire-retardant additives in polyurethane and polyester resins and as anti-foaming additives. None of the cyclic ketals derived from glycerol (Examples 1, 2, 3, 6, 9, 10, 11) are expected to be good humectants for use in inks, as they all have boiling points below 200 Cyclic ketals of this type, which are derived from polyhydric alcohols with higher molecular weights than glycerol, are expected to be better humectants than those derived from glycerol. However, in comparison to cosolvents used in current best practice inks, the compounds claimed in U.S. Pat. No. 3,741,986, are neither particularly good humectants nor particularly good penetrants.
In Japan Laid-Open Patent Application 64-13080, novel polyoxyalkylenated cyclic acetals derived from polyhydric alcohols and one or more aldehydes and novel polyoxyalkenylated cyclic ketals derived from polyhydric alcohols and one or more ketones are described. The aldehydes and ketones of that invention are hydrocarbon based and contain only carbon, hydrogen, and oxygen. Enumerated aldehydes include the following: formaldehyde, acetaldehyde, propionaldehyde, butyraldehyde, hexanal, benzaldehyde, etc. Enumerated ketones include the following: acetone, 2-butanone, 2-pentanone, 2-hexanone, 3-pentanone, 3-hexanone, 3-heptanone, 4-heptanone, 4-octanone, 5-nonanone, acetophenone, cyclohexanone, cyclooctanone, etc. The degree of oxyalkylenation ranges from 1 to 100 for non-acetalized or non-ketalized hydroxyl functionalities. The polyoxyalkylenated cyclic acetals and polyoxyalkylenated cyclic ketals of that invention have proposed applications as ink additives, although no specific examples of this usage are given.
For the case in which the polyoxyalkylene chain is short, the compounds of that invention have proposed applications as solvents in aqueous inks. For the case in which the polyoxyalkylene chain is long, the compounds of that invention have proposed applications as emulsifying agents. Several compounds of that invention, in which the polyoxyalkylene chains are short, are expected to be good humectants for use in ink, comparable to current best practice humectants. However, in comparison to cosolvents used in current best practice inks, the compounds claimed in Japan Laid-Open Patent Application 64-13080, are not particularly good penetrants.
In European Patent Application EP 0322110 A2, compounds similar to those described in Japan Laid-Open Patent Application 64-13080 are described. The main difference between the two sets of compounds is that those in EP 0322110 A2 have a terminal primary amine on the non-acetal/ketal end of the polyoxyalkylene chain. The compounds of that invention are useful as intermediates in the preparation of polyoxyalkylene-substituted colorants which may be reacted with condensation polymers. Specifically, the compounds are useful for preparing colored modifiers for polyurethane foams. Several compounds of that invention, in which the polyoxyalkenyl chains are short, are expected to be good humectants for use in ink. However, the primary amine functionality may be problematic, in that without neutralization, inks containing compounds of this type may have compatibility problems with adhesives used in commercial print heads. In any case, in comparison to cosolvents used in current best practice inks, the compounds claimed in European Patent Application EP 0322110 A2, are not particularly good penetrants.
As can be seen from the above examples, cyclic acetals, cyclic ketals, and their polyoxyalkylenated derivatives have been prepared for use as cosolvents in inks and in other applications. As can also be seen from the above, however, none is particularly good as an ink jet ink cosolvent, especially in regard to obtaining good reliability and a rapid drying rate. Accordingly, there remains a need for cosolvents for use in ink jet inks which provide reliable printing performance and are rapid drying. In other words, a need exists for cosolvents which function both as good humectants and good penetrants.